The photomasks are typically made from high precision, 0.060 inch thick soda-lime glass plates. Customarily, the clear glass is placed in a vacuum chamber, wherein a thin layer of chrome is evaporated onto its surface. A thin layer of photoresist is then applied, using well-known technique. The copy plate thus prepared is then contact printed from a master mask on a similar glass plate, generated by other means, all in usual fashion.
The basic purpose is to produce, from the master mask, precision copies to be used as working masks. Thus I may refer hereinafter to the master photomask, and to copy photomasks or copies which are to be produced in large number by contact printing from the master.
It is a general purpose of the invention to produce improved, high precision copy masks, in substantial number, at relatively high speed, particularly while avoiding the deterioration of the master mask that is commonly experienced.